/Engine/Source/Runtime/Slate/Public/Framework/Application/SlateApplication.h
Adds a modal window to the application. In most cases, this function does not return until the modal window is closed (the only exception is a modal window for slow tasks)
void AddModalWindow ( [TSharedRef](API\Runtime\Core\Templates\TSharedRef)< [SWindow](API\Runtime\SlateCore\Widgets\SWindow) > InSlateWindow, const [TSharedPtr](API\Runtime\Core\Templates\TSharedPtr)< const [SWidget](API\Runtime\SlateCore\Widgets\SWidget) > InParentWidget, bool bSlowTaskWindow )